Extended linear defects in scanned renditions of images on photographic elements, such as film, commonly occur. Such defects include, but are not limited to scratches, digs, processing draglines, coating streakiness, coating waviness, scanner defects, etc. For example, during the manufacture of photographic elements, defects in the coating process can lead to narrow regions, referred to as streaks, along the length of the photographic element in which one or more of the light-sensitive layers are affected. Because of the affected layer or layers, there is a change in the amount of light-sensitive material and/or coupler in the streak region. This manifests itself in abnormal characteristic data in the streak region. In a second example, a dirt particle in a camera can lead to a developable latent image formed by pressure sensitization when film is transported over the dust particle which is manifested as an extended linear defect.
The use of reference calibration patches exposed on a roll of film to enable better exposure control during optical printing is known in the art. See for example U.S. Pat. No. 5,767,983 issued Jun. 16, 1998 to Terashita entitled Color Copying Apparatus for Determining Exposure Amount from Image Data of an Original Image and a Reference Image. The use of reference calibration patches has also been shown to be useful in determining correction values for scanned film data used in digital printing. See for example U.S. Pat. No. 5,667,944 issued Sep. 16, 1997 to Reem et al. entitled Digital Process Sensitivity Correction; and U.S. Pat. No. 5,649,260 issued Jul. 15, 1997 to Wheeler et al. entitled Automated Photofinishing Apparatus. 
Although extended linear defects can lead to undesirable artifacts in images of scenes, the effects of such defects when they occur in reference calibration images containing sensitometrically exposed patches can be even more detrimental. If such defects can be detected and located, the location of the image defect can be fed into software intended to measure sensitometric patches, enabling such software to avoid using data derived from the defective region or apply appropriate reconstruction techniques to recover affected data.
In the prior art, U.S. Pat. No. 5,736,996 issued Apr. 7, 1998 to Takada et. al., entitled Image Reading Apparatus with a Function for Correcting Nonuniformity in Recording Density, and U.S. Pat. No. 5,189,521 issued Feb. 23, 1993 to Ohtsubo et. al. entitled Image Forming Apparatus and Method for Correction Image Density Non-Uniformity by Reading a Test Pattern Recorded by the Apparatus, describe methods of automatically detecting image nonuniformities by laying down a test target on a recording medium. However, the nonuniformites of interest are inherent to a recording head and not the presumably uniform medium onto which the image is recorded. Once a nonuniformity is detected, the recording head is automatically calibrated to deliver uniform densities to the recording medium despite nonuniformities in the recording head. This prior art fails to repair the nonuniformity when such defects arise in the recording medium rather than the recording device or occur after the recording step, as the location and severity of the defects in the medium cannot be determined at the time of printing reference calibration targets.
Standard techniques for locating linear objects in digitized images, illustratively image segmentation and description techniques as described in Digital Image Processing by Rafael Gonzalez and Paul Wintz, Addison-Wesley Publishing Company, Reading, Mass., 1977, may be applied to locate such defects in a nominally uniform sensitometric patch. However, in patches with exposures which exhibit low signal to noise ratios, such detection algorithms break down. Failure to detect a defect in such a patch could lead to bias in estimates of density or noise levels in the patch. Use of corrupted data in a calibration procedure could affect entire images in a deleterious fashion. Further, in highly structured nonuniform images such as two-dimensional barcodes, these standard techniques fail to reliably distinguish between linear features that are part of the barcode and those that arise from an artifact.
Accordingly there is a need for an improved method to detect and locate linear defects in scanned images of photographic elements.